This invention relates to a catalytic organic fuel processing apparatus and more particularly the apparatus as part of a fuel cell power system.
Fuel cell power systems have been used to supply power where an internal combustion engine is not practical, such as in manned space vehicles. Fuel cell power systems have also been proposed as electric vehicular power plants to replace internal combustion engines, however, various factors have limited their widespread use. Because space vehicle fuel cells used hydrogen as a fuel, and because gaseous hydrogen could not practically be stored in sufficient quantities aboard a vehicle, other fuels were examined as possible fuel cell anode feeds. However, alternative fuels had to be first converted into hydrogen which gave best fuel cell performance. This conversion step necessitated a fuel processing apparatus as part of a fuel cell based power plant. The fuel processing apparatus had to meet requirements for compactness and transient response for vehicular use. An especially rigorous requirement was that the fuel processing apparatus, after an extended shutdown period such as overnight parking, initiate the production of hydrogen for the fuel cells quite rapidly, if the apparatus was to be used in a motor vehicle. In addition, the fuel processing apparatus had to make rapid responses to changes in power demand while maintaining efficiency and low concentration of CO and other contaminates in the hydrogen produced for the fuel cells. Because the fuel processing reactions to produce hydrogen were often markedly endothermic, it was necessary to supply heat in widely varying quantities to the fuel processing apparatus to meet widely varying power demands. Often the fuel processing reactions could be catalytically enhanced, but the presence of a catalyst limited the maximum temperature at which the reactions could be run.
One of the more popular organic fuels proposed as a source of hydrogen for fuel cells was methanol. The overall reaction that converts methanol to hydrogen was: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
For the reaction 40.1 kcal/mol (of methanol) of heat was needed. Approximately two thirds of this heat was used for thermal input to obtain a 200.degree. C. reactant temperature. Industrial catalysts fabricated from partially reduced copper oxide and zinc oxide have been known to speed up the rate of reaction. However, a minimum of 15 to 20 minutes was normally necessary to bring the organic fuel processing apparatus up to temperature for the reaction to occur on a sustained basis.
U.S. Pat. No. 4,473,622 issued to Chludzinski et al. proposed a methanol-to-hydrogen cracking reactor for obtaining rapid system start up by a combination of direct and indirect heating of cracking catalyst. At system start up, the liquid methanol was burned and the hot combustion gases flowed over the outside walls of the catalytic chamber. The hot combustion gases were diverted back through the catalyst bed to heat the catalyst pellets directly. After operating temperature in the catalytic chamber for converting methanol was reached, the burner was switched from methanol to excess hydrogen from the fuel cells. Combustion gases were only in indirect heat exchange relationship with the catalyst chamber walls and were not circulated directly through the catalyst bed after operating temperature was reached. While this arrangement did reduce the start up time, it was necessary to control the temperature of the combustion gases circulating through the catalyst bed during start up to prevent damage to the catalyst by overheating. Additionally, response to transient high demand times during continuous operation was retarded by the necessity to switch fuels for combustion and to redirect the flow of combustion gases through the catalyst bed. Additionally, the flow of combustion gases through the catalyst bed risked the contamination and poisoning of the catalyst bed, especially where oxygen was present in the combustion gases. The necessity for a mechanism to control the temperature of the combustion gases circulating through the bed as well as the necessity for a valve mechanism to direct the combustion gases through the catalytic bed all added bulk to the fuel processing system. Finally, the temperature of the combustion gases circulating through the catalytic bed was hard to precisely control and damage to the catalyst from occasional bursts of overheated combustion gases was inevitable.
Overall, there is an existing need for an organic fuel processing apparatus capable of converting such fuels as methanol to hydrogen for use in fuel cells which has an acceptable start up time, efficiency, compactness, light weight, and controllable temperatures to prevent damage to any catalyst which is used to increase the conversion reaction rate. Additionally, the organic fuel processing apparatus must be able to meet transient demands for greatly increased output of organic fuel converted to the desired product such as hydrogen. Finally, a need still exists for a fuel cell power system which contains an organic fuel processing apparatus that can meet transient high power demands, has an acceptable start up time, and is compact and light weight enough for such uses as vehicle propulsion.